The development of the next-generation DVD (Digital Versatile Disc) has been in progress for the long-term recording of high-definition videos and for computer mass storage. In order to achieve a recording capacity four times or more than that of conventional DVDs, the wavelength of the semiconductor laser device must be in the 400 nm band rather than in the conventional 650 nm band. To this end, gallium nitride based materials are used.
The ridge waveguide structure is often used in both InGaAlP-based semiconductor laser devices in the 650 nm band and InGaAlN-based semiconductor laser devices in the 400 nm band. In this structure, a ridge portion to serve as a waveguide is formed in a cladding layer that is provided above the double heterojunction including an active layer, thereby confining horizontal transverse modes.
The next-generation DVD requires a gallium nitride based semiconductor laser device, which provides a high power for mass storage. In general, in gallium nitride based materials, magnesium (Mg) is used as impurities for providing the p-type conductivity. However, Mg is likely to form deep levels in gallium nitride based materials. Such deep levels formed in the vicinity of the active layer will promote non-emissive recombination between electrons and holes (JP 2002-111053A and its corresponding U.S. Pat. No. 6,617,606). This results in decreasing the emission efficiency of the semiconductor laser device and makes it difficult to meet the requirements for the next-generation DVD, which requires an optical power of 100 mW or more.